1. Field of the Invention
The present invention relates to a coordinate input apparatus used to control an externally connected computer or write characters and graphics by directly inputting coordinates on a display screen with a designation unit.
2. Related Background Art
Known examples of a conventional coordinate input apparatus are one for sensing a light spot on a screen using a CCD area sensor or linear sensor, and calculating and outputting coordinate values by image processing using barycentric coordinates or pattern matching, and one using a position detection element (analog device capable of obtaining an output voltage corresponding to a spot position) called a PSD (Position Sensitive Detector).
For example, Japanese Patent Publication No. 7-76902 discloses an apparatus for sensing a light spot of a parallel beam of visible light with a video camera to detect coordinates, and at the same time sending/receiving control signals by infrared diffusion light. Japanese Patent Application Laid-Open No. 6-274266 discloses an apparatus for detecting coordinates using a linear CCD sensor and a special optical mask.
Japanese Patent No. 2,503,182 discloses the arrangement of an apparatus using a PSD, and an output coordinate correction method therefore.
In resent years, the screen of a large-screen display has higher brightness, can be satisfactorily used even in a bright illumination environment, and is much in demand. Along with this, a coordinate input apparatus must be more resistant to disturbance light so as to be used in such an environment together with the large-screen display. Recently, devices using infrared rays as radio communication means increase in number. Since disturbance light is increasing in addition to infrared light and visible light, high resistance to disturbance light is one of the important characteristics of the apparatus.
However, as is apparent from Japanese Patent Publication No. 7-76902 and Japanese Patent Application Laid-Open No. 6-274266, an apparatus using a conventional CCD sensor can suppress disturbance light by only an optical filter. To the contrary, an apparatus using a PSD, like the one disclosed in Japanese Patent No. 2,503,182, can suppress the influence of disturbance light by frequency-modulating the light intensity and synchronously detecting the modulated wave. This apparatus is resistant to disturbance light with the use of an optical filter.
In a large-screen display, resolution is increased in addition to brightness. The resolving power of the coordinate input apparatus must also be increased, which cannot be attained in an apparatus using a PSD. This is because the dynamic range of a sensor output voltage directly corresponds to the input range. For example, when the entire screen is divided into 1,000 coordinates, the S/N ratio must be at least 60 dB or more. Further, since linear errors must be digitally corrected, as disclosed in Japanese Patent No. 2,503,182, a high-precision analog circuit, multi-bit AD converter, and arithmetic circuit are required. Since the S/N ratio of a sensor output signal depends on the light quantity and the sharpness of a light spot, only suppression of disturbance light is insufficient, and a bright, high-precision optical system is also required. Accordingly, the apparatus itself increases in cost and size.
As a method of increasing the resolving power using a CCD sensor, Japanese Patent Publication No. 7-76902 discloses simultaneous use of a plurality of video cameras. However, this increases the apparatus size and cost. Using one video camera having many pixels further increases the apparatus size and cost, compared to using a plurality of cameras. To attain a higher resolving power than the number of pixels by image processing, an enormous amount of image data must be processed at a high speed. To operate the apparatus in real time, its size and cost greatly increase.
In Japanese Patent Application Laid-Open No. 6-274266, a high resolving power is obtained by a special optical mask and signal processing. The resolving power can be increased so long as disturbance light is reduced and a high S/N ratio is ensured. In practice, in a linear sensor, a formed image is linear and cannot be separated from disturbance light within a plane, compared to an area sensor forming a point image. For this reason, the linear sensor is readily affected by disturbance light and can only be practically used in a specific environment almost free from disturbance light.